I've long thought that 20W50 is probably a bit on the heavy side - Austin recommended straight 30 grade for touring cars and 40 for sports. The higher grade for sports cars assumes that they will be run hotter, the heavier oil thus gives the same effective viscosity at the bearing at running temperature.
I measured my own oil temperature a while ago and it barely exceeded 65C (n.b. measured in the sump, a deep Nippy-style one) after an hour's run down country roads, so is getting nowhere near limits under 'normal' conditions.
Thus there is no obvious justification for using a 50 grade on a touring car today, especially on a rebuilt engine with a new Phoenix crank and machined big ends.
As others have observed, good flow is far more important, and that's where the multigrade wins hands down as 30 or 40 grade is porridge at room temperature. It may have been that consideration which held Austin back from specifying a 50 grade of course.
One must also take into account that multigrades, especially cheap ones, tend to break down with age, so if you are not a regular oil changer (my system is "constant loss") it's erring on the safe side. But - for example - a 10W40 really ought to be more than enough.
Regrettably the only way to really know these things is to test, and there are many owners here with happy and extensive experience of 20W50, which probably dates back to the days when 10W40 and lighter grades were not around. I myself have yet to quite pluck up the courage to move away from something which I know works well enough...though in time we may have to look at other options.
I measured my own oil temperature a while ago and it barely exceeded 65C (n.b. measured in the sump, a deep Nippy-style one) after an hour's run down country roads, so is getting nowhere near limits under 'normal' conditions.
Thus there is no obvious justification for using a 50 grade on a touring car today, especially on a rebuilt engine with a new Phoenix crank and machined big ends.
As others have observed, good flow is far more important, and that's where the multigrade wins hands down as 30 or 40 grade is porridge at room temperature. It may have been that consideration which held Austin back from specifying a 50 grade of course.
One must also take into account that multigrades, especially cheap ones, tend to break down with age, so if you are not a regular oil changer (my system is "constant loss") it's erring on the safe side. But - for example - a 10W40 really ought to be more than enough.
Regrettably the only way to really know these things is to test, and there are many owners here with happy and extensive experience of 20W50, which probably dates back to the days when 10W40 and lighter grades were not around. I myself have yet to quite pluck up the courage to move away from something which I know works well enough...though in time we may have to look at other options.